1. Field of the Invention
The present invention relates to fiber optic gyroscopes and more particularly pertains to a potting compound for use and fabrication of the fiber optic sensor coil of a gyroscope and method of making a potted fiber optic gyro sensor coil.
2. Description of Related Art
An interferometric fiber gyroscope includes the main components of 1. a light source, 2. two beam splitters, 3. a fiber optic sensing coil made of either polarization maintaining (PM) fiber or a low birefringence (standard telecommunications) fiber, 4. a polarizer (sometimes more than one) and 5. a detector for light from a light source is split by loop beam splitter into counter-propagating waves traveling in the sensing coil. The associated electronics measure the phase relationship between the two interfering counter-propagating beams of light that emerge from opposite ends of the coil. The difference between the phase shifts experienced by the two beams is proportional to the rate of rotation of the platform to which the instrument is fixed, due to the well known Sagnac effect.
Environmental factors can affect the measured phase shift difference between the counter-propagating beams, thereby introducing an error, such environmental factors include variables, such as temperature, vibration and magnetic fields. In general, such factors are unevenly distributed throughout the coil. These environmental factors induce variations in the optical light path that each counter-propagating wave encounters as it travels through the coil. The phase shifts induced upon the two waves are unequal, producing an undesirable phase shift which is indistinguishable from the rotation-induced signal.
Past approaches to reducing some of the sensitivities arising from environmental factors, have involved the use of potting compound, to pot the windings of the sensor coil within a matrix of an adhesive material. Such an approach is described in U.S. Pat. No. 5,321,593 for "Sensor Coil for Low Bias Fiber Optic Gyroscope", assigned to the assignee of the present application and U.S. Pat. No. 5,546,482 for "Potted Fiber Optic Gyro Sensor Coil for Stringent Vibration and Thermal Environments", also assigned to the assignee of the present invention, both disclosures being incorporated herein by reference.
Although, the approaches described in these patents have alleviated some of the sensitivity problems resulting from the noted environment factors, undesirable sensitivities still exist. The sensor electronics utilized with the fiber optic gyro sensor coil to measure the phase relationship between the two counter-propagating beams of light in the coil, generate heat. The heat from the electronics provide thermal gradients in the gyro assembly. When the rate of heating is constant the component temperature changes, but the thermal gradients themselves remain constant. However, when the rate of heating changes or fluctuates as it does during warm-up of the gyroscope and at times during thermal cycling, these gradients become time varying. These time varying gradients within the fiber optic gyro sensor coil, lead to false time varying inertial angle readings for the gyroscope. These false readings limit the sensitivity of the gyro, and tend to mask its performance.